It is known that aryl carbonates may be obtained by phase interface phosgenation (Schotten-Baumann reaction) of aromatic hydroxy compounds. Here, the use of solvents and caustic soda solution is a disadvantage because partial saponification of phosgene or chloroformate may take place due to the presence of alkali. In all cases, the large amount of sodium chloride which is produced is linked with pollution of the waste water. Furthermore, care has to be taken to recover the solvent, wherein effective protection of the environment must be ensured.
Therefore a condensation process which does not use solvents, in the presence of tetramethylammonium halides as homogeneous catalysts, has been suggested (U.S. Pat. No. 2,837,555). Here, however, the amounts of catalyst required are relatively large. As a rule 5-7 percent by weight of catalyst, with reference to the amount of phenol used, must be used in order to obtain economic rates of reaction. Reaction temperatures of 180.degree.-215.degree. C. are associated with the risk of decomposition of the thermally labile tetramethylammonium halides. Furthermore, the catalyst must subsequently be removed by washing with water, which makes its recovery very difficult. In addition, far more than the stoichiometric amount of phosgene is consumed.
According to another process (U.S. Pat. No. 3,234,263), diphenyl carbonates are obtained by heating phenyl chloroformates in the presence of large amounts of alkali/alkaline earth metal compounds or tertiary nitrogen bases as catalysts. This process has the disadvantage, however, that elevated temperatures are required and the catalysts such as alkali/alkaline earth metal compounds must be partially dissolved in order to achieve only approximately economically acceptable reaction times. In this process half the phosgene initially introduced is lost in the form of CO.sub.2. In addition, the chloroformate must be synthesized in a quite separate process step.
According to CA-A-2 058 359 (U.S. Pat. No. 5,167,946), diaryl carbonates are obtained by phosgenation of aromatic hydroxy compounds in the presence of aluminium compounds which are at least partially soluble under the reaction conditions or which are converted into soluble aluminium halides and obviously act as homogeneous catalysts in this form (cf. U.S. Pat. No. 2,362,865, col. 1, lines 45-53). This is why aluminium trichloride (solubility) is particularly preferred. Although very good yields are obtained, it is difficult to separate the catalysts from the products. In the case of distillation, account must be taken of the fact that these compounds have a certain volatility and that thermal decompositions may occur due to these aluminium compounds which lead to impurities, a reduction in quality and decreased yields. The same applies to the process in U.S. Pat. No. 2,362,865, which still mentions the use of titanium, iron, zinc and tin as metals or as their soluble salts, especially the chlorides and phenolates, as catalysts.
Thus, it appears sensible to use heterogeneous, insoluble catalysts which makes working up the reaction mixture a great deal easier. There have also been proposals relating to this. Thus, according to the disclosure in EP-A 516 355, aluminium trifluoride is particularly recommended, this being optionally applied to a carrier such as alumino-silicates. However, the synthesis of aluminium trifluoride involves With handling fluorine or hydrofluoric acid, which are very toxic compounds, and thus also complicated and expensive apparatus.